Adding to a list issue

Hello everyone,
I am trying to import objects in a Capella Model using Capella Studio based on an excel file.
However I have an unexpected issue for this. In a method I am trying to create an Operational activity and to add it to the tree of created Operational Activities based on its parent.
I have an if{}else{} to specify my destination of creation. If the parent’s ID is in the list of previously created Operational Activities then the destination is the parent otherwise it is the Root Operational Activity.
After defining the destination I created the Operational Activity using this line:
OperationalActivity currentOAct = OaFactory.eINSTANCE.createOperationalActivity(name);
And then I add the created Operational Activity in the list of children of my destination Operational Activity using this line:
destination.getChildrenOperationalActivities().add(currentOA ct);
However I get an error related to the list itself on the line where I add:
Caused by: java.lang.UnsupportedOperationException
For info the exact same process works perfectly fine on Modes creation. And in the case of Operational Activities it actually worked before but not anymore.
Thank you very much in advance